In recovering oil from subterranean deposits thereof, it is well recognized by persons skilled in the art and in the literature pertaining to oil recovery operations, that only a small percentage of the total oil present in these reservoirs is recovered by so called primary means, i.e., by employing natural drive energy present in the reservoir in the form of an underlying or edge water drive, gas cap or solution gas drive, etc. Even after application of waterflooding, commonly referred to as "secondary recovery", as much as half or more of the oil originally present in the reservoir remains in the reservoir.
Many prior art references describe the use of surfactant waterflood oil recovery methods for the purpose of increasing the amount of oil displaced by a drive fluid passing through a flow channel in a reservoir. Inclusion of surfactants of interfacial tension-reducing chemicals in the injected water achieves recovery from small capillaries and/or dead-end pore spaces not invaded by ordinary drive water, and so recovers additional oil from the reservoirs over that obtainable by waterflood. Petroleum sulfonates and other organic sulfonates, as well as more complex surfactants including alkyl- or alkylarylpolyethoxy sulfates and sulfonates have been disclosed in numerous prior art references for recovering additional amounts of oil over that recoverable by primary and secondary recovery techniques.
State-of-the-art surfactant waterflooding enhanced oil recovery methods frequently make use of a tracer fluid injected into one or more injection wells, whose presence can be easily detected in fluids being recovered from spaced-apart production wells. In employing tracer fluids in connection with surfactant waterflood oil recovery processes, the first measurement involves the arrival time, i.e., the elapsed time between injection of the fluid into the injection well and the time when the fluid containing the tracer material first arrives at one or more spaced-apart production wells. Simple qualitative analysis is adequate for determining the time when the fluid first arrives, and from this arrival time one can determine the volume of reservoir through which the injected fluid passes. This is one of the most important parameters which can be determined by use of tracer fluids, since it directly affects the amount of surfactant fluid which must be employed in a particular pattern, and aids in predicting the amount of additional oil one can expect from application of the enhanced oil recovery method. Once the fluid containing the tracer material has been detected at the production well, the more sophisticated techniques for monitoring and analyzing enhanced oil recovery processes require that the concentration of tracer material in the produced fluid be measured continually or intermittently over a period of time so the shape of the response curve (concentration versus time or fluid volume produced) can be determined.
Other uses for tracer fluids in connection with surfactant flooding or other enhanced oil recovery processes, include the ability to detect flow aberrations caused by pressure differentials in the reservoir created by factors other than the injection of enhanced recovery fluids, which distorts the pattern performance. The data obtained from tracer slugs employed before and/or with the surfactant fluid can also be relied on to explain and analyze results of a pilot field trial.
Tracers suitable for use in enhanced oil recovery must meet a number of requirements. They should be relatively inexpensive, and must be compatible with fluids naturally present in the reservoir and with the reservoir rock itself, as well as with fluids injected into the reservoir in connection with the enhanced oil recovery process. Moreover, the tracer material must be one which can be readily detected qualitatively and analyzed quantitatively in the presence of the materials naturally occurring in the formation fluids. For example, an aqueous sodium chloride solution could be utilized as a tracer but for the fact that most field brines contain sodium chloride in substantial quantities, and so detection and analysis of sodium chloride used as a tracer in the presence of naturally-occurring sodium chloride would be difficult.
Another requirement is that the tracer not be readily absorbed or otherwise removed from the tracer fluid, since more sophisticated analytical techniques require that the concentration of tracer material in the produced fluids be determined and compared with the concentration of tracer in the fluid injected into the injection well.
A serious problem has been encountered in the use of many tracers which are otherwise quite suitable, in that bacteria introduced into the fluid containing the tracer as a result of contamination during surface handling, or which naturally occur in oil field brines, attack and decompose the tracer materials. Since the tracer materials will be in the formation for relatively long periods of time, i.e., many months or even several years, it is essential that the tracer material be stable over relatively long periods of time at the temperature and other conditions existing in the petroleum reservoir.
In view of the foregoing discussion, it can be appreciated that there is a serious need for a method or additive for stabilizing tracer fluids against bacterial degradation. Ideally, the stabilizing additive should be one which is relatively inexpensive, and which is highly effective and long lasting in order to maintain the concentration of tracer material in the tracer fluid constant over long periods of time. Moreover, the additive must not interfere with analysis of the tracer material, and the stabilizing additive must not interfere or degrade the effectiveness of chemicals injected into the reservoir for the purpose of surfactant waterflooding enhanced oil recovery.
It is an object of this invention to provide an effective stabilizing additive for use in tracer fluids, and this objective is met in at least certain of the preferred embodiments to be described hereinafter below.